A subject I can probably help with.
The western coast of Britain was fairly gaeliziced by the Vth century, and probably earlier on. Several historians tend now to disregard migrations as myths, such as the foundation of Dal Riada or the exile of the Deisi. By this time travelling by sea on short distances from eastern Ireland to western Britain was maybe easier than travelling by land, and thus populations on either shores of the sea shared probably more than populations separated by mountains. This is particulary the case for Argyll. Populations living there may have been gaels since ages ago, at least far earlier than the Vth century when Dal Riada is supposed to have been founded. Of course populations travelled and move from one shore to the other, and elites established their power here and there. This should be more viewed as a continual flux for centuries than as "invasions" and brutal take-over of Pictish lands.
The wide repartition of Ogam inscribed stones in Wales (easternmost stone at Silchester!) show there was a good deal of Gaelic speakers there circa 500 AD. Interestingly, the local elites were both spoking Gaelic and Latin, and maybe Brittonic too. It is even thought by some scholars Ogam script originated in Britain, and was then bring to Ireland along with elements of Britain's Romano-christian culture. Most famous one was christianism, but there is also the pennanular broochs for exemple.
There is a good deal of discussion to do about both Deisi and Attecotti. Both seemed to be "inferior" tribes, forced to military service or chased from their lands. Both could be generic names for those tribes. So it is not unlikely some went to Britain to seek a better fortune, first in raids, but then also acting as foederati for the Romans and afterwards the Britons. There are several late roman units from the late IVth century
Notitia Dignitatum bearing the name of the Attecotti.
They were more likely coming to Britian as Pirate's because durning the 300s-400AD the Legion was no more and it was replaced with a Part-time millita and also with the empire ripping into The westen and eastern Empire, almost content civl war and the east stelling the best units of the army Britan was rip for Pirate's to start raiding the provinc and the Romano-Britrish just could not hold them and also the Anglo-saxons off
A bit off topic, Roman forces were still fairly active in IVth century Britain. Troops were withdrawed by Magnus Maximus in 383 AD, Stilicho circa 400 AD and Constantine III in 408 AD. The British usurpators probably recruited Gaels in their army.
I got several books on the subject. Will detail more if needed
